The present invention relates to an image reader for use with a digital copier or the like which uses charge coupled devices (CCDs) and other elements for reading an image on an original document as digital information.
Generally, a digital image reader uses a CCD or like image reading device for reading an image on an original document which is slit-exposed in a manner known in the art. Such a digital image reader allows images to be edited in a great number of different manners, compared to an analog copier and others. The prerequisite for a digital image reader to provide images which are comparable with those provided by an analog system is the increase in pixel density. Meanwhile, where a document to be read is a drawing or the like which is sized A1 or A0, for example, the number of pixels is so large that the pixels cannot be accommodated by a single CCD. Specifically, because the maximum number of pixels presently available with a CCD is 5000, for a pixel density of 16 pixels per millimeter, for example, only a document image which is about 300 millimeters wide can be accommodated. It follows that to read, for example, a A0 size document image whose width is 841 millimeters at a pixel density of 16 pixels per millimeter, three CCDs are required since the necessary number of pixels is 841.times.16=1845.
Usually, such a plurality of CCDS are mounted in line on a base plate. In order that a document image may be prevented from being lost in a joint portion of nearby CCDs, the nearby CCDs read a document image corresponding to the joint potion in an overlapping relation and, then, the image data ready by any of the two CCDs is cancelled. In this situation, it is necessary for each of the CCDs to be positioned with considerable accuracy on the base plate. However, because the base plate is usually implemented with a plain steel sheet, its temperature varies during operation of the image reader with the result that its dimensions are varied due to thermal expansion. The change in the dimensions of the base plate is reflected by a change in the ratio of the overlapping portion of a document image which corresponds to the joint portion of the nearby CCDs and is read by the CCDs in an overlapping relation, resulting in inaccurate image reading operations. Therefore, in an image reader of the type having a plurality of CCDs arranged in an array on a base plate, the positional relationship, i.e., distance between the nearby CCDs mounted on the base plate and data read by the CCDs have to be compensated as the dimensions of the base plate are changed by thermal expansion.